This invention relates to current amplifiers and more specifically to cascaded current mirror amplifiers.
A conventional current mirror is shown in FIG. 1 that is commonly found in both integrated and discrete amplifier and biasing circuitry. An input transistor 16 has a coupled base and collector to receive an input current at input terminal 10. An output transistor 18 has a base coupled to the base of input transistor 16. The collector of output transistor 18 provides the output current to output terminal 12. The emitter of transistor 16 and 18 are coupled together and to emitter terminal 14. Emitter terminal 14 is coupled to ground, a source of constant supply voltage, or to other circuitry. The gain of the current mirror shown in FIG. 1 is approximately unity. Alternatively, the current mirror of FIG. 1 may be regarded as an equivalent transistor having a current gain of approximately unity wherein terminal 10 is the base, terminal 12 is the collector, and terminal 14 is the emitter. Thus, terminal 10 is subsequently referred to as the amplifier base terminal, terminal 12 is referred to as the amplifier collector terminal, and terminal 14 is referred to as the amplifier emitter terminal in FIGS. 1-5.
The conventional method for obtaining current gain from a current mirror is shown in FIG. 2. In this circuit, output transistors 20 and 22 are coupled in parallel with output transistor 18. Therefore, the gain of the current amplifier is three, or alternatively, an equivalent transistor is shown having a current gain of three. However, as the current gain is increased by adding successive parallel output transistors, the corresponding -3 dB rolloff frequency decreases.
A cascaded current amplifier having improved frequency response characteristics is shown in FIG. 3A. In this circuit, the emitter current from a first current mirror including transistors 16 and 18 is returned to the input of a second current mirror including input transistor 24 and output transistor 26. Since the emitter current from the first current mirror is twice as large as the input current at amplifier base terminal 10, the size of transistors 24 and 26 is chosen to be twice as large as transistors 16 and 18. Since the collector of transistor 18 and transistor 26 are coupled together and to the amplifier collector terminal 12, the current gain of the amplifier shown in FIG. 3A is also three, or alternatively, an equivalent transistor is shown having a current gain of three. The bandwidth of the circuit shown in FIG. 3A is substantially improved over the circuit shown in FIG. 2 at the same gain.
The current mirror amplifier of FIG. 3A may be cascaded using several current mirrors, with the emitter current of the previous current mirror being directed to the input of the next current mirror. In such a configuration, each current mirror output current is summed, and device sizes are doubled in each succeeding current mirror. For N current mirror stages the total current gain is 2N+1.
The circuit of FIG. 3A, while representing an improvement in frequency response beyond the prior art circuit of FIG. 2, contains unnecessary connections that result in a less than optimal frequency response, difficulty of layout in an integrated circuit or etched circuit board, and potential current crowding if the devices used are not precisely matched. Therefore what is desired is a hierarchical current amplifier having fewer interconnections that would optimize frequency response, simplify layout, and reduce current crowding.